Stem cells exhibit an extraordinary ability to generate multiple cell types in the body. Besides embryonic stem cells, tissue specific stem cells serve a critical role during development as well as in homeostasis and injury repair in the adult. Stem cells renew themselves through proliferation as well as generate tissue specific cell types through differentiation. The characteristics of different stem cells vary from tissue to tissue, and are determined by their intrinsic genetic and epigenetic status. However, the balance between self-renewal and differentiation of different stem cells are all stringently controlled. Uncontrolled self-renewal may lead to overgrowth of stem cells and possibly tumor formation, while uncontrolled differentiation may exhaust the stem cell pool, leading to an impaired ability to sustain tissue homeostasis. Thus, stem cells continuously sense their environment and appropriately respond with proliferation, differentiation or apoptosis. It would be desirable to drive regeneration by controlling the timing and extent of stem cell proliferation and differentiation. Controlling the proliferation with small molecules that are cleared over time would allow for control of the timing and extent of stem cell proliferation and differentiation. Remarkably, tissue stem cells from different tissues share a limited number of signaling pathways for the regulation of their self-renewal and differentiation, albeit in a very context dependent manner. Some of these pathways are the Wnt and GSK3 pathways.
Lgr5 is expressed across a diverse range of tissues and has been identified as a biomarker of adult stem cells in a variety of tissues such as the gut epithelia (Barker et al. 2007), kidney, hair follicle, and stomach (Barker et al, 2010; Haegebarth & Clevers, 2009). For example, it was first published in 2011, that mammalian inner ear hair cells are derived from LGR5+ cells (Chai et al, 2011, Shi et al. 2012). Lgr5 is a known component of the Wnt/beta-catenin pathway, which has been shown to play major roles in differentiation, proliferation, and inducing stem cell characteristics (Barker et al. 2007).
Permanent damage to the hair cells of the inner ear results in sensorineural hearing loss, leading to communication difficulties in a large percentage of the population. Hair cells are the receptor cells that transduce the acoustic stimulus. Regeneration of damaged hair cells would provide an avenue for the treatment of a condition that currently has no therapies other than prosthetic devices. Although hair cells do not regenerate in the mammalian cochlea, new hair cells in lower vertebrates are generated from epithelial cells, called supporting cells, that surround hair cells.
Prior work has focused on transdifferentiation of supporting cells into hair cells through activation or forced expression of genes that lead to hair cell formation, with a particular focus on mechanisms to enhance expression of Atoh1 (Bermingham et al., 1999; Zheng and Gao, 2000; Izumikawa et al., 2005; Mizutari et al., 2013). Interestingly, cells transduced with Atoh1 vectors have been shown to acquire vestibular phenotypes (Kawamoto et al., 2003; Huang et al., 2009; Yang et al., 2012, 2013), and lack complete development. As mentioned, upregulating Atoh1 via gene insertion has been shown to create non-cochlear cell types that behave in a manner that is not found within the native cochlea. In addition, these methods increase hair cell numbers but decrease supporting cell numbers. Since supporting cells are known to have specialized roles (Ramirez-Camancho 2006, Dale and Jagger 2010), loss of these cells could create problems in proper cochlear function.
Thus, there remains a long felt need for new compounds that can preserve/promote the function of existing cells after injury.